Scottish Marine and Freshwater Science Vol 11 No 9
Scotland’s rivers support fish populations that are of high economic, conservation and cultural value. Evidenced based management of these important natural resources requires defensible, quantitative assessments of population status and trends. Electrofishing data are one of the most commonly collected sources of information on the status of salmon and other freshwater and diadromous fish populations providing status assessments at nested spatial scales ranging from individual sites (10s m) to the whole of Scotland. In 2018 the National Electrofishing Programme for Scotland (NEPS) was established to provide unbiased quantitative data on the abundance of Atlantic salmon and other freshwater fish species in Scotland’s rivers. When combined with a benchmark for expectation, this allowed assessment of the status of salmon stocks. In 2019, the survey was repeated. This report presents an analysis of the 2019 data, compares salmon abundance and status between years and reports on method developments since 2018. These include the introduction of a new capture probability model for trout, presentation of data on trout abundance and approaches for combining independent surveys within years, and repeat surveys between years. Climate conditions contrasted strongly between the NEPS survey years with 2018 recorded as one of the hottest and driest years on record, while 2019 was the second wettest summer ever recorded in Scotland. As a result wetted widths were on average 11-12% higher in 2019 than 2018. Salmon fry abundance declined by 44% between 2018 and 2019 in response to a substantial reduction in spawner numbers. Salmon parr numbers also declined, although the reduction was smaller at ca. 17% and some of this could reflect differences in wetted widths (higher wetted widths result in lower observed densities). The overall status of salmon at the national scale (based on fry and parr) declined from Grade 1 in 2018, to Grade 3 in 2019 (Grade 2 averaged across years). The number of NEPS regions classified as Grade 3 for salmon fry increased from 9 to 14 (of 27), although the regional performance of parr improved slightly (decreasing from 14 to 12) reflecting a stronger 2018 cohort year for fry. Future developments of the NEPS programme will include the introduction of a benchmark for trout and revisions to survey design to better reflect the spatial configuration of local management organisations, reporting needs, management of over-samples and improvements to the sample frame to reflect changes in the mapping of barriers, lochs and canals. Scientific challenges associated with spatial configuration of data providers and availability of suitable multi-pass data to parameterise capture probability models are identified. It is suggested that data providers obtaining funding for NEPS sampling in future years should be required to meet minimum specified criterion in terms of experience and multi-pass data provision.